Improving the Accuracy of Coliform Monitoring of Reclaimed Waters Used for Irrigation of Edible Crops.

Many water microbiology testing methods have been developed for use on relatively clean samples, but testing of reclaimed water quality can present challenges. With the growth in use of recycled water for irrigation of edible crops, accurate monitoring of water quality biomarkers, including Escherichia coli (E. coli), increases in importance. Chromogenic media, due to ease of use and rapid analysis turnaround, are widely used for detection of E. coli. The presence of unique levels of organic and inorganic compounds, however, alters the chemistry of reclaimed waters, potentially hindering accurate detection of E. coli using enzyme-based chromogenic technology. Our earlier work, using m-Coli Blue 24^® broth, revealed that false positive rates in E. coli identification in reclaimed water samples from a single treatment plant showed sharp seasonal differences, averaging 35% during summer, but rising to 75% in the late fall. Our current work extends this project to include analyses of reclaimed water produced in several treatment plants throughout Arizona. Using MI agar and molecular confirmatory methods, we have found that the rate of false positive identification of E. coli is much lower than with the m-Coli Blue 24^® broth, averaging only 10% in reclaimed water samples and 16% in irrigated soils over four seasons. To date, no false negatives (E. coli that are present but not detected) have been found. Sequencing of isolates reveals that Proteus and Aeromonas species are the most common bacteria contributing to false positive readings. False positive rates are highest (> 30%) in mid-summer and mid-winter, and correspond to periods of increased water electrical conductivity (> 1800 µS cm^-1), suggesting that dissolved chemical constituents decrease the accuracy of chromogenic media. We aim to identify the chemical changes in reclaimed water that hinder the performance of chromogenic media, to ultimately improve irrigation water quality monitoring programs.